Design and Fabrication of Darrieus Wind Turbine with Static Stress Analysis of Rotor & its Structures

Design and Fabrication of Darrieus Wind Turbine with Static Stress Analysis of Rotor & its Structures Sanjaya K. Mohapatra Department of Mechanical Engineering Jadavpur University D. K. Mondal Department of Mechanical Engineering Jadavpur University P. C. Roy Department of Mechanical Engineering Jadavpur University Abstract The objective of work is to design and build a self-staring vertical axis wind turbine that is capable of producing power in low velocity wind situations. The design of the turbine will include exploration of various self-starting options, as well as construction of both model and full-scale turbines. The full-scale turbine will be designed such that it can be connected to a generator and a torque transducer to measure the output power, rotational speed of the turbine. The design of the rotor and its structure have been made based on static stress analysis of three different material alloy steel, aluminum alloy and carbon steel. Based on the analysis, alloy steel has been chosen and design and fabrication have been made for further studies. 1. Introduction As the world continues to use up nonrenewable energy resources, wind energy will continue to gain popularity. A wind turbine is a type of turbomachine that transfers fluid energy to mechanical energy through the use of blades and a shaft and converts that form of energy to electricity through the use of a generator. Depending on whether the flow is parallel to the axis of rotation (axial flow) or perpendicular (radial flow) determines the classification of the wind turbine. [1]. 2. Wind Turbines Two major types of wind turbines exist based on their blade configuration and operation. The first type is the horizontal-axis wind turbine (HAWT). This type of wind turbine is the most common and can often be seen littered across the landscape in areas of relatively level terrain with predictable year round wind conditions. HAWTs sit atop a large tower and have a set of blades that rotate about an axis parallel to the flow direction. The second major type of wind turbine is the vertical axis wind turbine (VAWT). This type of wind turbine rotates about an axis that is perpendicular to the oncoming flow, hence, it can take wind from any direction. VAWTs consist of two major types, the Darrieus rotor and Savonius rotor. The Darrieus wind turbine is a VAWT that rotates around a central axis due to the lift produced by the rotating airfoils, whereas a Savonius rotor rotates due to the drag created by its blades. There is also a new type of VAWT emerging in the wind power industry which is a mixture between the Darrieus and Savonius designs. Recently, VAWTs have been gaining popularity due to interest in personal green energy. VAWTs target individual homes, farms, or small residential areas as a way of providing local and personal wind energy. This reduces the target individual's dependence on external energy resources and opens up a whole new market in alternative energy technology. Because VAWTs are small, quiet, easy to install, can take wind from any direction, and operate efficiently in turbulent wind conditions, a new area in wind turbine research has opened up to meet the demands of individuals willing to take control and invest in small wind energy technology [2]. The device itself is relatively simple. With the major moving component being the rotor, the more complex parts like the gearbox and generator are located at the base of the wind turbine. This makes installing a VAWT a painless undertaking and can be accomplished quickly. Manufacturing a VAWT is much simpler than a HAWT due to the constant cross-section blades. Because of the VAWTs simple manufacturing process and installation, they are perfectly suited for residential applications. The VAWT rotor, comprised of a number of constant cross-section blades, is designed to achieve 813

good aerodynamic qualities at various angles of attack. Unlike the HAWT where the blades exert a constant torque about the shaft as they rotate, a VAWT rotates perpendicular to the flow, causing the blades to produce an oscillation in the torque about the axis of rotation. This is due to the fact that the local angle of attack for each blade is a function of its azimuthal location. Because each blade has a different angle of attack at any point in time, the average torque is typically sought as the objective function. Even though the HAWT blades must be designed with varying cross-sections and twist, they only have to operate at a single angle of attack throughout an entire rotation. However, VAWT blades are designed such that they exhibit good aerodynamic performance throughout an entire rotation at the various angles of attack they experience leading to high time averaged torque. The blades of a Darrieus VAWT (D -VAWT) accomplish this through the generation of lift, while the Savonius-type VAWTs (S -VAWTs) produce torque through drag [4]. Both vertical axis wind turbines (VAWTs) and horizontal axis wind turbines (HAWTs) generate electricity from wind power. However, the VAWT is less efficient, less durable and does not work as well as the HAWT Comparison between VAWT and HAWT in different prospects are as follows. [2]. Wind blows faster when it is farther from the ground. VAWTs are usually built at ground level, making their overall energy output less than a HAWT, which is erected higher in the air. HAWTs use a propeller design to generate energy. The Propeller can be turned to face the wind. A VAWT uses the eggbeater shape so that it always faces the wind. However, although the VAWT can produce energy this way, efficiency is lost through not having its entire blade shape always against the wind. In the design for an HAWT, the greatest stress put on the blade by the wind is at the base of the blade, where it is strongest. In a VAWT design shaped like an eggbeater, the greatest stress is put on the center of the curved blades, where it is weakest. HAWTs rest on towers that hold them up. VAWTs may need guy cables running from their top to the ground, which can be impractical for farming areas. When replacing the rotor on an HAWT, it can be removed from the shaft. When replacing the rotor on a VAWT, the entire machine must be taken down. The Savonius turbine is one of the simplest turbines. Aerodynamically, it is a drag-type device, consisting of two or three scoops. Looking down on the rotor from above, a two-scoop machine would look like an "S" shape in cross section. Because of the curvature, the scoops experience less drag when moving against the wind than when moving with the wind. The differential drag causes the Savonius turbine to spin. Because they are drag-type devices, Savonius turbines extract much less of the wind's power than other similarly-sized lift-type turbines. Much of the swept area of a Savonius rotor may be near the ground, if it has a small mount without an extended post, making the overall energy extraction less effective due to the lower wind speeds found at lower heights. Savonius turbines are used whenever cost or reliability is much more important than efficiency. Most anemometers are Savonius turbines for this reason, as efficiency is irrelevant to the application of measuring wind speed. Much larger Savonius turbines have been used to generate electric power on deep-water buoys, which need small amounts of power and get very little maintenance. 3. DARRIEUS WIND TURBINE The Darrieus Wind Turbine is a vertical axis turbine or VAWT, which means that the main rotor shaft runs vertically. The Darrieus Turbine dates back to 1927, when French aeronautical engineer Georges Jean Marie Darrieus patented this innovative turbine design. The Darrieus Wind Turbine resembles a gigantic eggbeater and has two main advantages. The first main advantage is that the equipment including the gear box and the generator can be placed close to the ground. The second advantage to this type of wind turbine is that you don t need a new mechanism to turn the rotor against the wind. The Darrieus Wind Turbine is a lift-type vertical axis turbine and can function effectively no matter which way the wind is blowing. This wind turbine is powered by the lift forces that are created by a set of airfoils, which are the actual wing-shaped blades of the turbine. These allow the turbine to reach speeds that are higher than the actual speed of the wind, which makes them well suited to generating electricity. As the airfoils move forward through the air, the air flow creates an angle of attack that generates a force that gives a positive torque to the shaft of the turbine and helps it move in the direction it is already rotating. The same principles apply to a helicopter. That energy 814

coming from the torque and the speed of the airfoils is then converted into electricity. 4. Static Analysis of Turbine Structures 2.1 Force analysis of Turbine blade A snapshot of the cross-section of a D-VAWT blade can be seen in Fig. 1. As the blade rotates, the local angle of attack for that blade changes due to the variation of the relative velocity W. The induced velocity Vi and the rotating velocity ωr of the blade govern the orientation and magnitude of the relative velocity. This in turn changes the lift L and the drag D forces acting on the blade. As the lift and drag change both their magnitude and orientation, the resultant force F R changes. The resultant force can be decomposed into both a normal component F N and a tangential component F T. It is this tangential force component that drives the rotation of the wind turbine and produces the torque necessary to generate electricity. Due to the mechanism driving the rotation of a D-VAWT, it is possible that the blades can travel faster than the speed of the free stream velocity U, or in other words, the tip speed ratio at which these wind turbines operate is often higher than 1, leading to higher efficiency. [6]. Fig. 1 : The force analysis of blade element. The operating tip speed ratio (TSR) for a Darrieus rotor lies between 4 and 6. This design TSR then determines the solidity, gear ratios, generator speeds, and structural design of the rotor. The input specifications of the vertical axis wind turbine are as follows in the Table 1. Another model input was the NACA 0012 airfoil lift and drag coefficients. These coefficients were supplied for various Reynolds numbers, and were input into the model as a lookup table. Table 1.0 : Sizing spread sheet for model inputs. INPUTS Data Undisturbed Wind 6 m/s Speed Density of air 1.204 kg/m 3 Viscosity of air 1.81E-05 Ns/m 2 TSR 4 Solidity 0.15 Number of Airfoils 3 Blade Height 0.8 m Power required 50 watts Estimated Coefficient of 0.15 Performance The primary objective of the engineering analysis model was to understand the effects of blade pitching. Therefore, pitch angles were another required input for the model. It is known that during high rotational speeds, the most efficient operation of the turbine occurs when the pitch angle is zero, or stated another way; the angle of attack is 90º. As a result, blade pitching is only necessary during start up when large torques are needed. A range of pitch angles was chosen for the model analysis to determine how different angles affected performance. The range was input into the model as angles of attack that would be added to a pitch angle of 90º, and included angles of attack of -10, -5, -2, 0, 2, 5, 10, 12, 15, and 17. These 10 angles of attack resulted in pitch angles ranging from 80º to 107º. The reason for not choosing larger or smaller pitch angles was that eventually the blade would have to return to 90º. Angles further from the range chosen would involve a large angle change and would likely induce turbulent flow over the airfoil. [4,5]. After the inputs are specified, the next step of the analysis involves calculations. The first calculation was to determine the Reynolds number associated 815

with the flow over the airfoils. The equation for Reynolds number is given below as: where the average torque for N blades located at radius R from the axis of rotation is given by: Where, Re : Reynolds number W : wind speed (m/s) TSR(λ) : tip speed ratio ρ : density of air (kg/m 3 ) c : chord length (m) : viscosity of air (Ns/m 2 ) DETERMINATION OF F T, F N & AVERAGE TORQUE Once the actual angle of attack is determined, the model uses the Reynolds number along with the angle to lookup the appropriate lift (C l) and drag (C d) coefficients. When the lift and drag coefficients are determined, The normal (C n) and tangential (C t) coefficients can be calculated following equations, respectively. To find out the actual normal (F N) and tangential force (F T), C n and C t is multiplied by the dynamic pressure. Where, ρ : air density (kg/m 3 ) c : chord length (m) L : length of the turbine Blades (m) W : wind speed (m/s) It is important to note that represents the tangential force at only a single azimuthal position. Therefore, the process of determining α, C t, and F T must be repeated at all azimuthal locations before the torque can be calculated. Because F T is calculated at all azimuthal locations, it is said to be a function of θ and the average tangential force for a single rotation of one blade is: POWER AND EFFICIENCY The final step in predicting the performance of the wind turbine is determining the power it is able to extract from the wind and how efficiently it can accomplish that task. The amount of power the wind turbine is able to draw from the wind is given by: Therefore, the efficiency of the wind turbine is simply the ratio of the power produced by the wind turbine (P T) and the power available in the wind (P W) given by the expression: 5. Static Analysis of Rotor The resultant force (F R ) can be decomposed into both a normal component, F N and a tangential component, F T. It is this tangential force component that drives the rotation of the wind turbine and produces the torque necessary to generate electricity. So calculate the values as described in above force analysis section and apply these aerodynamic forces for analysis of the rotor of the turbine [6]. The wind turbine model draw in the Solid Works software, corresponding load and fixed constraints applied to the model at the same time. Three materials chosen for the wind turbine rotor, carbon fiber, aluminum alloy and structural steels were compared and analyzed. The properties of three materials are given in the table 3. Meshing of turbine rotor model and loading of turbine rotors for static stress analysis have presented in figure 2 & 3. The displacement simulation results of turbine rotor and static stress analysis of turbine rotor for the different material have been shown in figure 4 to figure 9. The triangular structure rotor maximum displacement and stress simulation results are shown. It is observed that the maximum stress is much less than the respective yield strength, not exceeding the allowable stress. The amount of deformation of the carbon fiber is 0.40945 mm and it is the smallest in the three materials. 816

Table 3 : The properties of the three materials International Journal of Engineering Technology Science and Research Fig. 2 : Meshing of turbine rotor model for static stress analysis Fig. 5 : Static stress analysis of turbine rotor by using carbon fiber. Fig. 3 : Loading of turbine rotors for static stress analysis Fig. 6 : The displacement simulation results of turbine rotor by using aluminum alloy. Fig. 4 : The displacement simulation results of turbine rotor by using carbon fiber. Fig. 7 : Static stress analysis of turbine rotor by using aluminum alloy. 817

Fig. 8 : The displacement simulation results of turbine rotor by using structural steel. Fig. 11 : The displacement simulation results of turbine rotor by using structural steel Fig. 9 : Static stress analysis of turbine rotor by using structural steel. Applying the aerodynamic forces F T and F N along with centrifugal force (C.F = mω 2 R) for analysis of the rotor of the turbine for structural steel maximum displacement and stress simulation results are shown in Fig. 10-12. The maximum stress is much less than the respective yield strength, not exceeding the allowable stress. The amount of deformation is 2.94 mm. Fig. 10 : Meshing of turbine rotor model for static stress analysis Fig. 12 : Static stress analysis of turbine rotor by using structural steel. Modal Analysis When the material is structural steel, it is the natural frequency of the ten modes as shown in Fig. 13 ( as sample results). It is observed that the ten orders of the natural frequencies are, 10.666, 11.57, 12.594, 15.334, 17.212, 31.304, 45.224, 46.173, 66.226 and 106.58 respectively. The natural frequencies are plotted in graph below in Fig. 14. The 1, 2, 3, 4 and 5 orders are similar. 6, 7 order natural frequency and 8 order natural frequencies are similar. This is due to the three groups of blades and the blade strut distributions are 120 and the first 3 order of their respective vibration modes in the XZ plane. 818

6. Design and Fabrication of Rotor Based on the above analysis, dimensions of the rotor and support structure have been designed and are prepared in 3-d model after static and modal analysis, using solid works with all details shown in figure 15. Fig. 13 : 1 st order modal vibration diagram. Fig. 14 : Natural frequencies n graph The rotational frequency of the wind turbine of three blades. In the variable speed wind turbine design, it must make sure that the speed of turbine blades does not close to in the first natural frequency range of the turbine model [3]. The rated speed of the wind generator model is 400 rev/min. According to the frequency formula: f = ω \ 2π Where, f = Rotational frequency in Hz. ω = Angular speed of rotor in rad \ s. And the above, the rotational frequency of the turbine rotor, f = 6 Hz. Since the natural frequency of the wind turbine model must be outside of ±10% of the wind turbine rotational frequency, it will not cause resonance. So it can be guideline to fix the design parameters. Fig. 15 : 3-d model of wind turbine using solid works. The fabrication of the turbine blade, rotor and support structure are done as per the manufacturing drawing produced by solid works using structural steel, which involves various operations like plate cutting, bending to shape, welding, turning, facing, drilling, assembly and painting etc. The turbine blade is made out of 2mm M.S. sheet, which involves various operations like cutting of sheet as per sheet development of drawing, bending to the aerofoil shape as per drawing. The turbine blade band is also made out of 2mm M.S. sheet, which involves operations like cutting of sheet as per sheet development of drawing, bending to the shape as per drawing. The turbine blade arms is made out of 5mm M.S. plate, which involves operations like cutting of plate as per plate development drawing, bending to the shape and drilling as per drawing. The rotor shaft is machined in lathe for its desired 819

finished which involves various operations like turning, facing etc. shown in figure 16. International Journal of Engineering Technology Science and Research Fig. 16 : Turning operation of shaft in lathe. The other components like bearing housing, rotor disc, base plates, bearing covers are lying in the works as semi finished conditions after marking and waiting for drilling tapping operations shown in Fig. 17) and the Fig. 18 shows the installation of vertical axis wind turbine in Fluid mechanics laboratory, Mechanical Engineering Department of Jadavpur University for demonstration and testing. Fig. 17 : Bearing housing, rotor disc, base plates, bearing covers are lying in the works. Fig. 18: The Vertical axis wind turbine is installed at Fluid mechanics laboratory, Mechanical Engineering Department of Jadavpur University. 7. Conclusions The design of the rotor and its structure have been made based on static stress analysis of three different material alloy steel, aluminum alloy and carbon steel. Based on the analysis, alloy steel has been chosen and design and fabrication have been made. The developed wind turbine will be designed such that it can be connected to a generator and a torque transducer to measure the output power, rotational speed of the turbine. REFERENCES [1 ] www.wikipedia.org/wiki/vertical_axis_wind_tu rbine. [2 ] www.energy-withoutarbon.org//files/wind/vertical-axis-windturbines. [3 ] Yuyi, Z., Decai, Z., Liyang, L. and Jun, L. The design of vertical axis wind turbine rotor for Antarctic, Information Technology Journal, 12, 2013, 604-613. [4 ] Small-scale vertical axis wind turbine design by Javier Castillo, Bachelor s Thesis Tampere University of Applied Sciences, 2011. 820

[5 ] Vertical Axis Wind Turbine by Jon DeCoste, Denise McKay, Brian Robinson, Shaun Whitehead, Stephen Wright, Mechanical Engineering Design project, Dalhousie University, 2005 [6 ] Aerodynamic shape optimization of a vertical axis wind turbine by Travis justin carrigan, MS Thesis in Aerospace Engineering, The University of Texas 2010 821